1. Field of the Invention
The invention relates to a method and installation for producing biconical wire, i.e., wire having repeating cylindrical wire sections, on the one hand, and conical wire sections, on the other hand, from a cylindrical starting material, especially for production of helical compression springs.
2. Description of Related Art
In automotive engineering, helical compression springs are used extensively as axial springs. The prevailing trend in automotive engineering toward minimizing the weight of the automobile has increasingly resulted in suppliers being confronted with the demand that individual components must be designed as lightweight as possible--with a given function and lifetime. With helical compression springs used as axial springs, this has led to the use of high-strength wires with a tensile strength of 2000 MPa or more. Furthermore, weight savings can be achieved with helical compression springs that are used as axial springs by using biconical wire to produce them, i.e., wire with repeating cylindrical wire and conical wire sections (see, published unexamined German Patent Applications Nos. 41 29 172 and 42 33 462 as well as German Patent No. 196 04 408). Moreover, special spring characteristics can be achieved by using biconical wire for production of helical compression springs (see, column 1, lines 26 through 36 of published unexamined German Patent Application No. 41 29 172). Further more a minimization of the required fitting space for the corresponding helical compression spring can be realized.
Whenever biconical wire has been used, so far, to produce helical compression springs, the biconical wire has been produced by a process of paring from wire with a constant diameter, i.e., from a cylindrical starting material. On the one hand, this is relatively expensive from a production standpoint; in addition to the cost of materials and personnel costs, there is also the additional cost of materials due to material losses. On the other hand, the paring operation causes striations or scratches on the surface of the wire; such paring striations or scratches can lead to a local increase in stress, and consequently, to a reduced lifetime, or at least a greater range of scattering in lifetimes.
Moreover, in conjunction with improving the properties of martensitic wires for production of helical compression springs by cold rolling, it is also known (see published unexamined German Patent Application No. 42 33 462, column 9, lines 25 through 43, FIGS. 6a and 6b) that a rotating, program-controlled cold rolling device may be provided for forming the martensitic wire to an inconstant diameter after cutting to predetermined lengths, thus yielding inconstant resistance moments. However, this is the state of the art only on paper as it has not led to a biconical wire suitable for production of helical compression springs.